Case Study On

Distributed Systems Security

by Merv Wagner

Historical context: the evolution of security needs

1965-75 1975-89 1990-99 Current

Platforms Multi-usertimesharingcomputers

Distributed systemsbased on localnetworks

The Internet, wide-area services

The Internet + mobiledevices

Sharedresources

Memory, files Local services (e.g.NFS), local networks

Email, web sites,Internet commerce

Distributed objects,mobile code

Securityrequirements

User identification andauthentication

Protection of services Strong security forcommercialtransactions

Access control forindividual objects,secure mobile code

Securitymanagementenvironment

Single authority,single authorizationdatabase (e.g. /etc/passwd)

Single authority,delegation, repli-cated authorizationdatabases (e.g. NIS)

Many authorities,no network-wideauthorities

Per-activityauthorities, groupswith sharedresponsibilities

Goals

System security policy– Cost-benefit - Baseline

Primary objectives– Confidentiality– Integrity – Availability

Secondary objectives– Authentication – Non-repudiation

Other concerns– Access control - Audit trail – Timely service - Record keeping – Anonymity

Secure channels

Properties Each process is sure of the identity of the other Protection against repetition and reordering of data

Employs cryptography Secrecy based on cryptographic concealment Authentication based on proof of ownership of secrets

Cryptographic concealment is based on: Confusion and diffusion Conventional shared crypto keys Public/private key pair

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Principal A

Secure channelProcess p Process q

Principal BThe enemyCryptography

Definitions

Asymmetric algorithm – public/private key cryptography that utilizes a trap-door function, such as RSA

Availability - system services are available when they are needed

Certificate - an electronic document containing a statement signed by a principal

Confidentiality - secrecy, information is only disclosed according to security policy

Integrity - information is not destroyed or corrupted and the system performs data operations correctly

DES - Data Encryption Standard adopted by National Bureau of Standards in 1977; famous cryptosystem that uses a 56-bit secret key

Digital Signature - emulates a conventional signature by using cryptography to verify to a third party that a message or document is produced by the signer

RSA - Rivest, Shamir, Adleman encryption algorithm; first public-key cryptosystem; based on the use of trap door function of two very large prime numbers

Symmetric algorithm – secret-key cryptography that uses a common key and a one-way encryption function, such as DES

Alice and Bob share a secret key KAB.

1. Alice uses KAB and an agreed encryption function E(KAB, M) to encrypt and send any number of messages {Mi}KAB to Bob.

2. Bob reads the encrypted messages using the corresponding decryption function D(KAB, M).

Alice and Bob can go on using KAB as long as it is safe to assume that KAB has not been compromised.

FeaturesScenario 1: Secret communication with shared secret key

Issues:

Key distribution: How can Alice send a shared key KAB to Bob securely?

Freshness of communication: How does Bob know that any {Mi} isn’t a copy of an earlier encrypted message from Alice that was captured by Mallory and replayed later?

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Bob is a file server; Sara is an authentication service. Sara shares secret key KA with Alice and secret key KB with Bob.

1. Alice sends an (unencrypted) message to Sara stating her identity and requesting a ticket for access to Bob.

2. Sara sends a response to Alice. {{Ticket}KB, KAB}KA. It is encrypted in KA and consists of a ticket (to be sent to Bob with each request for file access) encrypted in KB and a new secret key KAB.

3. Alice uses KA to decrypt the response.

4. Alice sends Bob a request R to access a file: {Ticket}KB, Alice, R.

5. The ticket is actually {KAB, Alice}KB. Bob uses KB to decrypt it, checks that Alice's name matches and then uses KAB to encrypt responses to Alice.

FeaturesScenario 2: Authenticated communication with a server

A ticket is an encrypted item containing the identity of the principal to whom it is issued and a shared key for a communication session.

This is a simplified version of the Needham and Schroeder (and Kerberos) protocol.

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Bob has a public/private key pair <KBpub, KBpriv>

1. Alice obtains a certificate that was signed by a trusted authority stating Bob's public key KBpub

2. Alice creates a new shared key KAB , encrypts it using KBpub using a public-key algorithm and sends the result to Bob.

3. Bob uses the corresponding private key KBpriv to decrypt it.

(If they want to be sure that the message hasn't been tampered with, Alice can add an agreed value to it and Bob can check it.)

FeaturesScenario 3: Authenticated communication with public keys

Mallory might intercept Alice’s initial request to a key distribution service for Bob’s public-key certificate and send a response containing his own

public key. He can then intercept all the subsequent messages.

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Alice wants to publish a document M in such a way that anyone can verify that it is from her.

1. Alice computes a fixed-length digest of the document Digest(M).

2. Alice encrypts the digest in her private key, appends it to M and makes the resulting signed document (M, {Digest(M)}KApriv) available to the intended users.

3. Bob obtains the signed document, extracts M and computes Digest(M).

4. Bob uses Alice's public key to decrypt {Digest(M)}KApriv and compares it with his computed digest. If they match, Alice's signature is verified.

FeaturesScenario 4: Digital signatures with secure digest function

Digest is a compressed form of the plaintext message

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FeaturesX509 Certificate format

Subject Distinguished Name, Public Key

Issuer Distinguished Name, Signature

Period of validity Not Before Date, Not After Date

Administrative information Version, Serial Number

Extended Information

Certificate: a statement signed by an appropriate authority.Certificates require:

• An agreed standard format• Agreement on the construction of chains of trust• Expiry dates, so that certificates can be revoked.

Certificates as credentials

Certificates can act as credentials– Evidence for a principal's right to access a resource

The two certificates shown could act as credentials for Alice to operate on her bank account– She would need to add her public key certificate

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FeaturesCertificates

1. Certificate type: Account number2. Name: Alice3. Account: 62626264. Certifying authority: Bob’s Bank5. Signature: {Digest(field 2 + field 3)} KBpriv

Alice’s bank account certificate

Public-key certificate for Bob's Bank

1. Certificate type: Public key

2. Name: Bob’s Bank

3. Public key: KBpub

4. Certifying authority: Fred – The Bankers Federation

5. Signature: {Digest(field 2 + field 3)} KFpriv

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StructureCryptographic Algorithms

Symmetric (secret key)E(K, M) = {M}K D(K, E(K, M)) = M

Same key for E and DM must be hard (infeasible) to compute (one-way function) Usual form of attack is brute-force: try all possible key values

Asymmetric (public key)Separate encryption and decryption keys: Ke, Kd

D(Kd. E(Ke, M)) = Mdepends on the use of a trap-door function to make the keys. E has high

computational cost. Very large keys > 512 bits

Hybrid protocols - used in SSL (now called TLS)

Uses asymmetric crypto to transmit the symmetric key that is then used to encrypt a session.

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Message M, key K, published encryption functions E, D

StructureCipher block chaining and stream ciphers

n

n+3 n+2 n+1 XOR

E(K, M)

n-1n-2n-3

plaintext blocks

ciphertext blocks

Cipher block chaining (CBC)

XOR

E(K, M)number generator n+3 n+2 n+1

plaintext stream

ciphertext stream

buffer

keystream

Stream cipher

Most algorithms work on 64-bit blocks.Weakness of simple block cipher:- repeated patterns can be detected.

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Symmetric encryption algorithms

These are all programs that perform confusion and diffusion operations on blocks of binary data

DES: The US Data Encryption Standard (1977). No longer strong in its original form. 56-bit key

Triple-DES: applies DES three times with two different keys. 112-bit key

AES: A proposed US Advanced Encryption Standard (1997). 128/256-bit key.

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Asymmetric encryption algorithms

RSA: The first practical algorithm (Rivest, Shamir and Adelman 1978) and still the most frequently used. Key length is variable, 512-2048 bits.

Asymmetric algorithms are ~1000 x slower and are therefore not practical for bulk encryption, but their other properties make them ideal for key distribution and for authentication uses.

Depend on the use of trap-door functionsA trap-door function is a one-way function with a secret exit - e.g. product of two large numbers; easy to multiply, very hard to factor

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ImplementationDigital signatures

Requirement:– To authenticate stored document files as well as messages– To protect against forgery– To prevent the signer from repudiating a signed document (denying

their responsibility)

Encryption of a document in a secret key constitutes a signature- Impossible for others to perform without knowledge of the key- Strong authentication of document- Strong protection against forgery- Weak against repudiation (signer could claim key was compromised)

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ImplementationSecure digest functions

- Encrypted text of document makes an impractically long signature- So we encrypt a secure digest instead

- Secure digest function computes a fixed-length hash H(M) that characterizes the document M

- H(M) should be:

- Fast to compute

- Hard to invert

- MD5: Developed by Rivest (1992). Computes a 128-bit digest

- SHA: (1995) based on Rivest's MD4 but made more secure by producing a 160-bit digest

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Implementation Digital signatures with public keys

Signing

hH(doc)

D(Kpub,{h}) h'

h = h'?authentic:forgedVerifying

M

H(M)

128 bits

h E(Kpri, h){h}Kpri

M

signed doc

M

{h}Kpri

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Implementation Low-cost signatures with a shared secret key

Signing

Verifying

M

K

M

K

h = h'?authentic:forged

h

M

signed doc

H(M+K) h

h'

H(M+K)

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Signer and verifier share a secret key K

MAC: Message Authentication Code

Performance of encryption and secure digest algorithms

Key size/hash size(bits)

Extrapolatedspeed

(kbytes/sec.)

Optimized speed

(kbytes/s)

TEA 128 700 -

DES 56 350 7746

Triple-DES 112 120 2842

IDEA 128 700 4469

RSA 512 7 -

RSA 2048 1 -

MD5 128 1740 62425

SHA 160 750 25162

Algorithm

Publickey

Secret key

Digest

speeds are for a Pentium II processor at 330 MHZ

ApplicationKerberos authentication and key distribution service

Secures communication with servers on a local network– Developed at MIT in the 1980s to provide security across a large

campus network > 5000 users– Based on Needham - Schroeder protocol

Standardized and now included in many operating systems– Internet RFC 1510, OSF DCE– BSD UNIX, Linux, Windows 2000, NT, XP, etc.– Available from MIT

Kerberos server creates a shared secret key for any required server and sends it (encrypted) to the user's computer

User's password is the initial secret shared with Kerberos

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System architecture of Kerberos

ServerClient

DoOperation

Authenticationdatabase

Loginsession setup

Ticket-granting

service T

Kerberos Key Distribution Centre

Serversession setup

Authen-tication

service A1. Request for

TGS ticket

2. TGSticket

3. Request forserver ticket

4. Server ticket5. Service request

Request encrypted with session key

Reply encrypted with session key

Servicefunction

Step B

Step A

Step C

C S

ApplicationSecure Socket Layer (SSL)

Key distribution and secure channels for internet commerce– Hybrid protocol; depends on public-key cryptography– Originally developed by Netscape Corporation (1994)– Extended and adopted as an Internet standard with the name Transport

Level Security (TLS)– Provides the security in all web servers and browsers and in secure

versions of Telnet, FTP and other network applications

Design requirements– Secure communication without prior negotiation or help from 3rd parties– Free choice of crypto algorithms by client and server– communication in each direction can be authenticated, encrypted or both

SSL protocol stack

SSLHandshake

protocol

SSL ChangeCipher Spec

SSL AlertProtocol

Transport layer (usually TCP)

Network layer (usually IP)

SSL Record Protocol

HTTP Telnet

SSL protocols: Other protocols:

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negotiates cipher suite, exchanges certificates and key masters

changes thesecure channel to a new spec

implements the secure channel

ClientA

ServerB

ClientHello

ServerHello

SSL handshake protocol

Establish protocol version, session ID, cipher suite, compression method, exchange random start values

Certificate

Certificate Request

ServerHelloDone

Optionally send server certificate and request client certificate

Certificate

Certificate VerifySend client certificate response if requested

Change Cipher Spec

Finished

Change Cipher Spec

Finished

Change cipher suite and finish handshake

SSL record protocol

Application data abcdefghi

abc def ghiRecord protocol units

Fragment/combine

Compressed units

Compress

MAC

Hash

Encrypted

Encrypt

TCP packet

Transmit

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Significance

Distributed systems require increased security

Goal of confidentiality, integrity and availability

New systems must build security in during design

Cryptographic algorithms are means to achieve distributed security– Secret-key (symmetric) - DES– Public/private key (asymmetric) – RSA

Need for increased bit length of keys to maintain cryptographic security

Digital signatures/Certificates

Kerboros/Secure Socket Layer

More Internet communications will be secure in the future

Summary

Essential to protect the information in distributed systems

Achieved by access control mechanisms and secure channels

Public-key and secret-key cryptography provide the basis for authentication and secure communication

Kerberos and SSL are widely-used system components that support secure and authenticated communication

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References

Anderson, James P.; "Computer Security Technology Planning Study"; ESD-TR-73-51, Vol I; Electronic Systems Division, Air Force Systems Command; Hanscom AFB, MA; Oct 1972.

Brassard, Gilles; Modern Cryptology A Tutorial; Springer-Verlag; New York; 1988.

Coulouris, George; Dollimore, Jean; Kindberg, Tim; "Teaching Material Slides"; Based on: Distributed Systems Concepts and Design; Pearson Education Ltd; Harlow, England; 2001.

Coulouris, George; Dollimore, Jean; Kindberg, Tim; Distributed Systems Concepts and Design; Pearson Education Ltd; Harlow, England; 2001.

Lampson, B.W.; "Dynamic Protection Structures"; AFIPS Conf. Proc. 35; FJCC; 1969, pg 27-38.

Langsford, Alwyn; Moffett, Jonathan D.; Distributed Systems Management; Addison-Wesley; Wokingham, England; 1993.

Moreno, April L.; "Distributed Systems Security: Java, CORBA, and COM+"; SANS Institute Information Security Reading Room; http://www.sans.org; Sept 14, 2002.

Summers, Rita C.; Secure Computing Threats and Safeguards; McGraw-Hill; New York; 1997.

Weissman, Clark; "Security Controls in the ADEPT-50 Timesharing System"; AFIPS Conf. Proc. 35; FJCC; 1969, pg 119-133.